The present invention relates to a low-viscosity two-component polyurethane system for casting an expanded (i.e., porous) polytetrafluoroethylene structure in conventional manufacturing equipment.
The prior art describes two component polyurethane systems for casting, i.e., potting, molding or encapsulating. Solvent-free polyurethane casting systems are useful for embedding electrical parts, for sealing the ends of cables, for producing homogeneously soldered accumulators, and in grouting compositions.
The components are not mixed until reaction is desired. On mixing the isocyanate and hydroxyl-containing components react to form the polyurethane product. Non-reactive components can be incorporated in one of the reactive components.
One type of two component polyurethane system consists of a polyol component and an isocyanate component; the other type consists of a prepolymer component and a curative component. The prepolymer is usually excess isocyanate reacted with a lesser amount of polyol with more of the same or a different polyol as the curative component; alternatively the prepolymer is excess polyol reacted with a lesser amount of isocyanate and the curative component is more of the same or a different isocyanate. These systems can be used to make articles comprising a flexible or rigid polyurethane, a supported or unsupported expanded polytetrafluoroethylene structure, and optionally a housing.
An example of a commercial system is a polymeric methylene diphenylene diisocyanate (polymeric MDI) and polypropylene glycol system marketed under the name "CONOTHANE EN-21" (registered trademark of Conap, Inc.) which has a mix viscosity of 2000 centipoise (cps.) and an isocyanate-polyol mix ratio of 1 to 1.16. Another example is a polyether prepolymer end capped with toluene diisocyanate and a polyether curative component, designated "PRC 1564" and available from Products Research and Chemical Corp., which has mix viscosity of 8000 cps. and a prepolymer-polyol mix ratio of 1 to 0.08. These systems illustrate the high final viscosities usually associated with prepolymers. These systems are useful in potting and encapsulation applications, for example, as electrical insulation for electrical assemblies. "Low viscosity" is a relative term. West German Pat. No. 3,500,722 (1986) describes a material having a viscosity of 9600 cps. at room temperature. Viscosities in the ranges described above are sometimes referred to as "low". However, in certain demanding applications where wetting and penetration of a substrate which has a very fine microstructure must occur, these polyurethane casting systems are not satisfactory. A working viscosity an order of magnitude lower is required for good flow characteristics.
West German Patent Application No. P 31 35 672.9, filed September 9, 1981, and published March 24, 1983, relates to the application of low-viscosity, easily castable mixtures which self-cure to polyurethane at low temperatures (under 50.degree. C. ) for intermediate layers in laminated safety plates. The mixtures are characterized by the fact that the viscosity of both the individual components and the entire mixture lies below 600 cps./25.degree. C. and that the mixture consists of:
(a) linear propylene oxide ethers ranging in molecular weight from 800 to 4000 (preferably from 1000 to 2500) that may contain up to 15 mol. % ethylene oxide units, PA1 (b) multifunctional propylene oxide polyethers ranging in molecular weight from 250 to 1250 (preferably from 400 to 800) that may contain up to 15 mol. % ethylene oxide units, PA1 (c) cycloaliphatic diisocyanates, PA1 (d) 0.05 to 2.5 percent by weight of organic tin catalysts in the mixture and, if necessary, PA1 (e) conventional additives, with the ratio of (a) to (b) being 1:1 to 16:1 and the NCO/OH ratio (c)/(a) +(b) measuring 0.8 to 1.3.
In these formulations, significant amounts of trifunctional glycols are used such that the resulting polyol blends have hydroxyl functionalities greater than 2.5.
The disadvantage of using a trifunctional compound, such as that used in German Patent Application No. 31 35 672.9, is early gellation. In addition, the resulting polymers have long cure times (e.g., 4 hours), and are very soft having a Shore A hardness of 10-80.
As used herein, low viscosity is defined as equal to less than 500 cps. at room temperature when measured as described hereafter. When viscosities this low are required, many polyurethane casting systems which utilize prepolymers or high molecular weight components cannot be used because of their inherently higher viscosities.
When viscosities less than 500 cps. are required, particularly when the preferred viscosity of less than 200 cps. is required, certain inherent difficulties are present. Low molecular weight polyols are very susceptible to moisture absorption and contamination. When the polyol is contaminated, even by small amounts of water, bubble-free castings cannot be produced since the water reacts with the isocyanate and releases carbon dioxide. Current production procedures often require daily degassing of the polyol components which is inconvenient, time consuming, and expensive.
Water absorption is a problem common to all polyols having high hydroxyl numbers, i.e., polyols having a high hydroxyl number. Water solubility in di- and trifunctional polyols declines with increasing molecular weight, and it is higher for trifunctional than difunctional polyols. For example, with difunctional polypropylene glycols having molecular weights of 425, 760, 1000, and 2000 the water solubilities are 100, 0.2, 0.23, and 0.17% (weight of water per weight of polyol), respectively. For example, with trifunctional polypropylene glycols having molecular weights of 260, 725, 1000, 2000, and 5000 the water solubilities are 100, 25, 22, 4 and 2.5% , respectively. The trifunctional polypropylene glycols of West German patent application No. P 31 35 672.9 are 25 to 100% water-soluble.
Another constraint which arises in formulating these systems is the ratio in which the components must be mixed. Mixing can be done most simply and economically using a single motor to drive two identical gear pumps, piston pumps, or diaphragm pumps to meter the two components into the mixing apparatus. Ratios other than 1:1 may require significantly more complicated and expensive equipment, such as two synchronized variable speed motors to drive two separate pumps of different capacities if the required ratio deviates significantly from 1 to 1.
There is therefore a need for a low viscosity two component polyurethane casting system with good wetting properties, preferably a system which can be used at a 1 to 1 volume ratio.